Defense Secretary Robert M. Gates is creating a department-wide task force to focus on ways to counter the roadside bombs that have caused 80 percent of U.S. casualties in Afghanistan.

The challenges are different from those in Iraq, Gates told reporters Thursday before a visit to a Wisconsin factory that is producing a rugged new armored vehicle for use in Afghanistan. He said most of the improvised explosive devices, or IEDs, in Iraq are based mainly on artillery shells and are triggered electronically. Those in Afghanistan, he said, are made primarily from fertilizers such as ammonium nitrate, with mines as detonators.

He also pointed out that Afghanistan's terrain is different, its road system is different -- streets running from paved to unpaved to nonexistent -- and the bomb builders' networks are structured differently than in Iraq.

"I have decided I need to focus my attention on this problem," he said.